Optical storage disks are known as recording medium for video, audio, or digital information, and consist of a substrate with a coating into which optically scannable indicia in concentric or spiral-shaped tracks are generated. These indicia alter the optical characteristics (e.g. intensity or polarization) of a light beam of low intensity which is directed onto the rotating storage disk for reading out the stored information. For writing in the indicia, light beams of higher intensity can be used which alter an optical characteristic of the coating layer, or injection molding processes in those cases where the indicia represent small recesses, or so-called pits.
The advantage of optical storage disks lies in the high storage density that can be achieved, since optical scanning permits track widths and track spacings in the micrometer range.
Such small dimensions require a servo-control of the light beams focussed onto the information tracks, such servo-control being usually carried out with pre-grooved guide tracks on the optical storage disk. If, to give an example, the focus of the light beam is displaced during the rotation of the disk relative to the guide track, the intensity distribution in the reflected light beam will be altered, too, which is determined with suitably arranged photodetectors (e.g. 4-quadrant detectors) and converted into a corresponding closed-loop control signal.
In conventional optical storage disks the required relief of pre-formed guide tracks (or guide grooves) is generated in a synthetic layer which is either part of the synthetic substrate forming the entire optical storage disk (video or audio disks) or which has been deposited on a glass substrate, as suggested mainly for high quality optical storage disks for the storing of digital data. Onto the guide track relief an additional layer can then be applied which, e.g. for magneto-optical storage disks, represents the actual information storage layer.
The guide grooves are made preferably with stamps in the synthetic layer, e.g. a polymeric layer; owing to the small dimensions of the structures, the large surface to be processed with a perfect finish, and the high costs involved, photolithographic methods do not appear desirable for commercial use.
The characteristics of glass, its resistance to environmental influences and a low optical double refraction recommend it for substrates of high-quality optical storage disks. However, the hitherto used sandwich structure with a deposited synthetic layer in which the guide tracks are formed with the necessary precision would increase the costs for the manufacture of such disks. If eraseable storage disks are used whose information can be erased and modified by light beams of high intensity, e.g. in disks with a thermomagnetic storage layer, the repeated thermal load would cause decomposition of the synthetic layer and reduce the disk lifetime. Besides, the relatively low breaking resistance of glass represents a problem which calls for a complex chemical hardening process.